I plan to use subprocess.Popen (in Python 3.11.2) to implement git mktree < foo.txt. Now I face the question.

In order to reproduce my situation, here is the script that creates the environment.

#!/bin/bash

export GIT_AUTHOR_NAME=foo
export [email protected]
export GIT_AUTHOR_DATE="Tue Jun 4 21:40:15 2024 +0800"
export GIT_COMMITTER_NAME=foo
export [email protected]
export GIT_COMMITTER_DATE="Tue Jun 4 21:40:15 2024 +0800"

rm -rf foo
git init foo
mkdir -p foo/hello
echo hello > foo/hello/hello.txt
echo hello > foo/hello.txt
echo world > foo/world.txt
git -C foo add .
git -C foo commit -m 'hello world'
git -C foo log --no-decorate
git -C foo ls-tree HEAD hello.txt
git -C foo ls-tree HEAD world.txt

It’s expected to print the commit and 2 blob entries.

commit d2b25fd15c1435f515dd6379eca8d691dde6abeb
Author: foo <[email protected]>
Date:   Tue Jun 4 21:40:15 2024 +0800

    hello world
100644 blob ce013625030ba8dba906f756967f9e9ca394464a    hello.txt
100644 blob cc628ccd10742baea8241c5924df992b5c019f71    world.txt

I want to create a commit from the tree that has only hello.txt and world.txt.

import subprocess

# get the blob entry of hello.txt
o, e = subprocess.Popen(
    ['git', 'ls-tree', 'HEAD', 'hello.txt'],
    stdout=subprocess.PIPE,
    env={'GIT_DIR': 'foo/.git'},
).communicate()
line1 = o.decode()

# get the blob entry of world.txt
o, e = subprocess.Popen(
    ['git', 'ls-tree', 'HEAD', 'world.txt'],
    stdout=subprocess.PIPE,
    env={'GIT_DIR': 'foo/.git'},
).communicate()
line2 = o.decode()

# write the 2 lines to foo.txt
with open('foo.txt', 'w') as f:
    f.write(line1)
    f.write(line2)

# create a new tree object from foo.txt
with open('foo.txt') as f:
    o, e = subprocess.Popen(
        ['git', 'mktree'],
        stdin=f,
        stdout=subprocess.PIPE,
        env={'GIT_DIR': 'foo/.git'},
    ).communicate()
    tree = o.decode()
    print(f'created tree object {tree}')

I wonder if I can use an in-memory file object so that I don’t have to create and remove foo.txt. As io.StringIO is recommended in many answers, I try the code.

import subprocess
import io

line1 = '100644 blob ce013625030ba8dba906f756967f9e9ca394464athello.txtn'
line2 = '100644 blob cc628ccd10742baea8241c5924df992b5c019f71tworld.txtn'

with io.StringIO(line1 + line2) as f:
    o, e = subprocess.Popen(
        ['git', 'mktree'],
        stdin=f,
        stdout=subprocess.PIPE,
        env={'GIT_DIR': 'foo/.git'},
    ).communicate()
    tree = o.decode()
    print(f'created tree object {tree}')

It raises an exception.

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "C:Python311Libsubprocess.py", line 892, in __init__
    errread, errwrite) = self._get_handles(stdin, stdout, stderr)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:Python311Libsubprocess.py", line 1339, in _get_handles
    p2cread = msvcrt.get_osfhandle(stdin.fileno())
                                   ^^^^^^^^^^^^^^
io.UnsupportedOperation: fileno

According to the io doc, it seems io.StringIO does not have a file descriptor.

fileno()

Return the underlying file descriptor (an integer) of the
stream if it exists. An OSError is raised if the IO object does not
use a file descriptor.

Is there any in-memory file object that has a file descriptor? Or is there any method to bypass the exception with io.StringIO in subprocess.Popen?

Thanks for any help.